In his book An Anthropologist on Mars, neurologist Oliver Sacks
tells the story of his patient Virgil, a man with blinding cataracts. In the beginning of the story, Virgil is
in his early fifties, and he has just had his cataracts removed. His
retinas are absorbing their first images in nearly half a century. When
Sacks visits Virgil, he finds someone who is trying to learn to see.
Virgil could see color and bold motion immediately after his surgery;
more subtle aspects of vision, however, continued to elude him.
Sacks writes:

"Virgil's cat and dog bounded in to greet and check us--and
Virgil, we noted, had some difficulty telling which was which. This comic
and embarrassing problem had persisted since he returned home from
surgery: both animals, as it happened, were black and white, and he kept
confusing them--to their annoyance--until he could touch them,
too...Further problems became apparent as we spent the day with Virgil. He
would pick up details incessantly--an angle, an edge, a color, a
movement--but would not be able to synthesize them, to form a complex
perception at a glance. This was one reason the cat, visually, was so
puzzling: he would see a paw, the nose, the tail, an ear, but could not
see all of them together, see the cat as a whole."

When this book was published in 1995, the story was extremely rare; very
few cases like Virgil's had ever been documented. Vision restoration in an
adult has not become much more common in the past nine years. Certain
emerging technologies, however, are hinting that some visual repair is
possible. Scientists and surgeons are slowly learning how to remove
constraints on the eye's ability to see; unleashing the brain's ability to
see is another story. In the September 2003 issue of Nature
Neuroscience, neuroscientist Ione Fine, at the University of
California in San Diego, and her colleagues reported a case study similar
to Virgil's. Their subject, whom they call "MM," was blinded in his right
eye and lost his left eye completely when he was only three and a half
years old. The cause of right eye blindness was chemical and thermal
damage to the cornea, the clear tissue that covers the central part of the
eye, including the pupil and the colored iris. When MM was still a child,
doctors attempted to transplant a healthy cornea into his damaged right
eye, but the transplant was unsuccessful. There was too much damage to his
limbus, an area of the eye next to the cornea that is essential for proper
corneal functioning.

Growing up, MM could perceive a small amount of light in his environment
(as could Sacks's patient Virgil), but he had no form or contrast
perception whatsoever. He would notice if the room he was in suddenly
changed from dark to bright, but he could not discern the shapes of
objects in his surroundings, nor could he tell where one object ended and
another began.

Forty years after he was blinded, surgeons performed a stem cell
transplant in MM's right eye. Stem cells are the least developed type of
cell; they have not yet chosen what type of adult cell to become. They
can be coaxed by chemicals in their cellular environments to become
whatever type of cell the body area needs, and they can then integrate
themselves into existing tissue. MM's physicians implanted stem cells from
another person's healthy cornea and limbus. Soon, these new stem cells
began to grow and move into MM's cornea, where they developed into mature
corneal cells, replacing his old, damaged cells.

Immediately after this surgery, MM reported that he could see simple
shapes. Today, he seems to have essentially no deficits in recognizing
simple forms. He can easily report the orientation of a bar: vertical,
horizontal, or any diagonal in between. And he can consistently identify
orientation changes in a bar, even if the variations between bars are very
slight. Immediately post-surgery, he also began to see and recognize
colors very easily. These developments alone are amazing in a person who
had seen nothing but vague impressions of light and dark for 40 years. As
Fine as her co-workers continued to test MM's sight, however, certain
deficiencies became evident. Although his ability to detect contrast at
low spatial frequencies -- if the contrasting pieces were each very large
-- was nearly as good as the ability in people with normal eyesight, his
ability to detect high spatial frequencies -- contrasting colors in very
thin stripes -- was substantially impaired. If presented with a panel like
the square farthest to the left:

MM would most likely detect both blue and black bars. But, if presented
with the square farthest to the right, he would probably be unable to
distinguish purple bars from black ones. MM's eyesight reveals shortfalls
in vision tasks that others find easy, such as picking coherent patterns
out of random noise, or seeing that line segments could form one straight
line if they were connected:

Both circles are made from white dots, but one pattern is random, while
the other is circular. Which one looks spiral? Most people easily choose
the circle on the right, but MM cannot answer this type of question.

Among these random line segments, is there a group that could be connected
to form one long line? MM does not see these types of patterns, either.

MM also does not easily identify implied shapes. For example, most of us
would probably say that both images below contain a white square in the
middle. MM only sees the square in the picture on the right. If the
square's outline is missing, he is unable to interpret its implied
presence:

He has incredible difficulty identifying complex shapes (like most objects
that we encounter in day-to-day life) and faces. By far the most difficult
tasks for MM involve three-dimensional interpretation of his environment.
When an image is projected onto the retina, it is two dimensional, because
the retina is essentially flat. When we are very young, our brains learn
to use depth cues, such as shadows and line perspective, to see the
three-dimensional world. Eventually, incorporating these cues into a
coherent picture of the world becomes involuntary.

Our ability to judge size correctly is one example of the brain's
reinterpretation of two-dimensonal images. When a person walks away from
us, the image of her becomes smaller and smaller on our retina. We know
that people do not actually shrink as they move away, however. The brain
combines the shrinking retinal image with perspective and depth cues from
the surroundings, and we "decide" that the person is moving away.

When MM lost his sight when he was three years old, his brain probably had
not yet constructed the connections that incorporate separate perceptions
into one combined perception. When a person walks away from MM, he has to
remind himself that the person is not actually shrinking in size! MM's
difficulties with three-dimensional interpretations are also obvious from
his explanations of drawings.

For example, when normally sighted
people are presented with this drawing:

They say that it is a flat depiction of a 3D cube. MM sees the above
drawing as a "square with lines."

He also does not have an easy time with transparent images. If
presented with this:

MM says that the drawing contains three shapes,
side by side, rather than two overlapping shapes, one of which is
transparent.

Additionally, he does not automatically integrate shading cues into his
perceptions of objects. People who have had normal eyesight since birth
usually say that, in the below drawing, the middle shape in the bottom row
is caving inward (concave), and the rest of the shapes are protruding
outward (convex).

We interpret these drawings in this way because our brains assume that a
light source (maybe the sun) is coming from above. So if the shape is
brighter on the top, that part must be protruding outward, into the sun's
rays. If the shape is brighter on the bottom, then the top is in shadow
and therefore caving inward, out of the sun's light. The researchers
report that MM can sometimes answer a problem like this correctly, but
that he seems to be reasoning explicitly about the light source and the
resulting shadows, rather than seeing the objects' shapes instantly and
automatically.

Post-surgery, physicians examined MM's right retina for any abnormalities.
They found no retinal degeneration, and his retina's electrical responses
were normal. His retina did not seem to be the cause of his vision
deficiencies. In fact, his problems didn't seem to be vision deficiencies
so much as visual interpretation deficiencies. And deficiencies of this
sort lie not with the retina's ability to perceive light and color, but
with the brain's ability to process the retina's signals correctly. We
usually do not think of the above problems as involving interpretation,
because we have performed these interpretations so many times, and from
such a young age. But since MM lost his sight at an early stage of
development, since he had no visual input into his brain after age three,
the researchers suspect that the visual centers in his brain did not
develop normally -- and now, they likely never will.

The Visual System. Courtesy of George Mather, University of Sussex.

There is a window of opportunity in youth, often called a critical
period, during which the brain can best form neural connections that
correspond both to retinal images and to practical experience. During the
critical period for the visual cortex, normal visual input is required to
wire everything correctly. If input is missing during this period, the
brain's links will probably not be built correctly. In fact, brain tissue
ordinarily used in visual processing might even be taken over by other
systems, perhaps tactile or olfactory systems.

Some of MM's visual abilities lend further support to the theory that he
missed a critical period of visual development. He is quite good at visual
tasks that involve motion. Tasks that stumped him at first often became
solvable if motion was incorporated into them. He became able to detect
the circular patterns in random noise if the patterns were moving. And he
began to see the "square with lines" as a cube if the lines moved, and the
cube appeared to be rotating.

At the end of their evaluations, the researchers saw some patterns
emerging in MM's visual abilities and deficiencies. His ability to
detect and identify simple form, color, and motion is essentially normal.
His ability to detect and identify complex, three-dimensional forms,
objects, and faces is severely impaired. The researchers have a tentative
explanation for these variations in visual skill.

Motion processing develops very early in infancy compared with form
processing. By the time MM lost his eyesight in the accident, the motion
centers in his brain were probably nearly complete. So when he regained
some eyesight in his forties, those connections in the brain were ready to
go. The parts of the brain that process complex shapes, however, do not
develop until later in childhood, so MM's brain likely missed its chance
to establish those particular brain connections. The authors also propose
that our brains may retain the ability to modify and refine complex form
identifications throughout life, not just throughout childhood. New
objects and faces are continually encountered throughout life, and our
visual processing centers must be able to adapt and learn to see new
shapes and forms. MM's brain never had the chance to learn.

MM's visual capacities continue to improve, but he also remains
somewhat uncomfortable with his new sense. As a blind person, MM became
extremely proficient at skiing, with the help of a guide to give him oral
directions. After his eyesight was restored, skiing frightened him.
The trees, snow, slopes, people -- everything whizzed by him, chaotic and
uninterpretable. After much practice, he is now a moderate sighted
skiier -- but when he really wants to go fast and feel confident, he
closes his eyes.

Even long after his cataract surgery, Virgil seemed to prefer to identify
and characterize objects by their feel. Sacks watches him one day has he
runs his hands over a statue:

"Exploring it swiftly and minutely with his hands, he had an
air of assurance that he had neer shown when examining anything by sight.
It came to me -- perhaps it came to all of us at this moment -- how
skillful and self-sufficient he had been as a blind man, how naturally and
easily he had experienced his world with his hands, and how much we were
now, so to speak, pushing him again the grain: demanding that he renounce
all that came easily to him, that he sense the world in a way incredibly
difficult for him, and alien."

Although Virgil, MM and others like them certainly possess a
rudimentary form of vision, decades of visual deprivation may never
be completely redeemable. The human brain has an amazing capacity for
plasticity, but there are some things that it cannot do. MM will
likely never see the way that we see.

"UCSD Study on How Newly Sighted Blind People Learn to See Provides
Clues to Development of Visual System." University of California, San
Diego, August 24, 2003:
http://ucsdnews.ucsd.edu/newsrel/soc/sightregained.htm

Storkey, A. "Visual Illusion of the Month." Institute for Adaptive
and Neural Computation, University of Edinburgh:
http://www.anc.ed.ac.uk/~amos/visualillusion.html